This invention relates to flow control in communication devices. More specifically it relates to a method for flow control in asymmetric digital subscriber line devices.
Many applications used from digital computers are asymmetric. For example, video on demand, Internet access, intranet access, remote local area network access, multimedia access, are specialized services that typically require high data rates downstream, but have relatively low data rates demands upstream.
An Asymmetric Digital Subscriber Line (xe2x80x9cADSLxe2x80x9d), is a communications technology that transmits an asymmetric data stream over a conventional twisted pair of telephone wires. An Asymmetric Digital Subscriber Line transmits a larger data rate downstream to a subscriber from a telephone switching office than from a subscriber back to the telephone switching office. Asymmetric Digital Subscriber Lines typically transmit about 1.5 Mega bits-per-second (xe2x80x9cMbpsxe2x80x9d) to about 9 Mbps downstream to a subscriber, and about 16 kilo-bps (xe2x80x9ckbpsxe2x80x9d) to 640 kbps upstream back to a telephony switching office. The asymmetric data rates depend on a number of factors, including length of the twisted pair of copper wires, copper wire gauge, and cross-coupled interference.
ASDLs are described in several standards including those by the American National Standards Institute (xe2x80x9cANSIxe2x80x9d), the International Telecommunication Union (xe2x80x9cITUxe2x80x9d) and others. For example, see xe2x80x9cNetwork and Customer Installation Interfaces - Asymmetric Digital Subscriber Line (ADSL) Metallic interface,xe2x80x9d ANSI-T1.413-1995, or xe2x80x9cAsymmetrical digital subscriber line (ADSL) transceiver,xe2x80x9d ITU-G.992.1-1999. Information about these standards can be obtained on the Internet at the Universal Resource Locators (xe2x80x9cURLsxe2x80x9d) xe2x80x9cwww.ansi.orgxe2x80x9d and xe2x80x9cwww.itu.chxe2x80x9d respectively. In addition, more information can be obtained on ADSLs from the ADSL Forum at the URL xe2x80x9cwww.adsl.com.xe2x80x9d
An ADSL system typically comprises two asymmetric devices connected by a conventional twisted pair of copper wires. An ADSL Transmission Unit-Central (xe2x80x9cATU-Cxe2x80x9d) is a device at one end of an ADSL connection at a telephony switching office. An ADSL Transmission Unit-Remote (xe2x80x9cATU-Rxe2x80x9d) is a device at another end of an ADSL connection at a subscriber or customer site.
The ATU-R may be integrated into a service module. A service module typically converts received digital signals into signals suitable for particular subscriber or customer premise equipment. The ATU-C may be integrated within an access node. An access node typically includes digital loop carrier systems such as a Digital Subscriber Line Access Multiplexer (xe2x80x9cDSLAMxe2x80x9d) concentrating individual voice lines to T1 lines, cellular antenna sites, Private Branch Exchanges (xe2x80x9cPBXsxe2x80x9d), Optical Network Units (xe2x80x9cONUsxe2x80x9d) or other carrier systems.
ADSL systems typically use Discrete Multi-Tone Modulation (xe2x80x9cDMTxe2x80x9d) for data transmission. Low speed services, such as Plain Old Telephone Service (xe2x80x9cPOTSxe2x80x9d), are carried in a baseline modulation frequency or low frequency, while higher speed multimedia services are modulated at higher frequencies.
Some applications require transport of packet data. An Asynchronous Transfer Mode (xe2x80x9cATMxe2x80x9d) system can use high speed services on ADSL systems as a physical layer to transport data packets. As is known in the art, ATM is a high-speed packet transmission mode. ATM segments and multiplexes data traffic into small, fixed-length units called xe2x80x9ccells.xe2x80x9d A cell is 53-octects, with 5-octects for the cell header, and 48-octects for the cell data. ATM provides four services classes that may use constant bit-rates, variable bit-rates, available bit-rates and unspecified bit-rate services. The four ATM service classes can be used to provide Quality-of-Service (xe2x80x9cQoSxe2x80x9d) functionality.
An ADSL system typically includes one or more xe2x80x9csplitters.xe2x80x9d Splitters are filters that separate high frequency and low frequency ADSL signals. A splitter may be integrated into ATU-C or ATU-R, physically separated from the ATU-C or ATU-R, or divided between high pass and low pass functionality, with the low pass functionality physically separated from the ATU-C or ATU-R.
An ADSL system can also be used without splitters. The International Telecommunications Union""s spilterless standard includes xe2x80x9cSplitterless Asymmetrical Digital Subscriber Line Transceivers,xe2x80x9d ITU-G.992.2-1999. The Splitterless ADSL standard is also called xe2x80x9cG.lite.xe2x80x9d G.lite is a new standard that uses DMT baseline modulation delivering a maximum downstream bandwidth of up to 1.4 Mbps, but is less sensitive to noise and other transmission problems.
There are several problems associated with using regular ADSL or Splitterless ASDL. Conditions may change on an ADSL connection. For example, a telephone may go on-hook or off-hook or noise may change conditions on the twisted pair of copper wires used for the ADSL connection. The change in condition may result in a degradation of signal quality on the connection. In addition, the change in condition may result in a degradation of a Quality-of-Service is being used (e.g., an ATM Quality-of-Service).
ADSL devices typically use a xe2x80x9cFast Retrainxe2x80x9d procedure to adapt ADSL transmission characteristics to changing line conditions. As is known in the art, the Fast Retrain procedure includes quickly adjusting to line conditions including amplitude response, delay distortions, timing recovery and echo characteristics in an ADSL transmission system.
However, the result of the xe2x80x9cFast Retrainxe2x80x9d procedure may be unable to sustain a Quality-of-Service as a lower speed traffic rate could be higher than that of a newly negotiated higher speed ATM traffic rate after recovery. Out-of-balance traffic rates will increase ADSL connection utilization, which in turn will discard data (e.g., ATM cells) and cause data retransmission. The retransmission may further decrease lower and higher speed traffic to throughput.
Thus, it is desirable to have a procedure to adapt ADSL transmission characteristics to changing line conditions using spliterless ADSL (i.e., G-lite). The procedure should adapt to changing ADSL line conditions without decreasing traffic throughput using splitterless ADSL as well as regular ADSL.
In accordance with preferred embodiments of the present invention, some of the problems associated with changing line conditions on ADSL connections using are overcome. A method for ADSL flow control is presented.
The method includes detecting a change on a first asymmetric network device on a connection between the first asymmetric network device and a second asymmetric network device in an asymmetric data transmission system. The detection includes but is not limited to, detecting any of a noise signal, onhook/offhook signal, ringing signal, or other signal. A data rate adaptation procedure is started to adapt the first data transmission rate based on the changed condition on the connection. A flow control procedure is invoked on the first asymmetric network device. The flow control procedure buffers outbound data and notifies an application layer on the first asymmetric network device to temporarily cease data transmission at the first data transmission rate. A second data transmission rate is invoked on the connection between the first asymmetric network device and the second asymmetric network device based on the change on the connection. The data transmission rate is adjusted on the first asymmetric network device to use the second data transmission rate on the connection between the first asymmetric network device and the second asymmetric network device.
Adjusting the data transmission rate includes notifying the application layer on the first asymmetric network device to resume data transmission at the second data transmission rate. Adjusting the data transmission rate may also include changing one or more data transmission characteristics used for the first data transmission rate before transmitting any data at the second data transmission rate.
The present invention may help adapt ADSL transmission characteristics to changing line conditions without decreasing data traffic throughput. The present invention may be especially useful for maintaining quality-of-service transmissions with a lower transmission rate, using splitterless ADSL (e.g., G.lite).
The foregoing and other features and advantages of preferred embodiments of the present invention will be more readily apparent from the following detailed description. The detailed description proceeds with references to the accompanying drawings.